This invention relates in general to semiconductor devices and, in particular, to MOSFET devices and processes for forming them.
Trench MOSFET structures are particularly useful for low-voltage power MOSFET devices. An existing dense trench structure 10 is shown in FIG. 1. The structure uses self-alignment technology of the trench etch and for recessing the polysilicon 22 within the etching. The dense trench MOSFET 10 includes a drain metal 30 on one surface of an N+ substrate 26. A lightly doped N-type epitaxial region 24 is grown on the substrate 26. The epitaxial layer receives a p-well diffusion 20 followed by N+ implants that form source regions 14. A P+ body region 17 is provided between the source implants that form source regions 14. A source metal 12 contacts the N+ source regions 14 and the P+ body region 17. A trench structure includes a sidewall oxide 16 that lines the trench. Within the trench there is a highly conductive layer of polysilicon 22. The polysilicon layer is covered with a dielectric, typically borophosphorosilicate glass (BPSG) 18. In operation, when a voltage is applied to the polysilicon gate electrode 22, the current flows in a vertical direction between the source regions 14 and the drain 30 along the channel adjacent the sidewalls of the trench.
The structure 10 shown in FIG. 1 provides for a relatively dense trench structure in the MOSFET power device. The structure uses self-alignment technology for the trench etch and for the polysilicon recess etch. Thereafter, the device is subject to an etching of the BPSG. That etch ensures that there will be enough material removed to establish a good connection to the gate 22 as well as to the source contact 14.
The structure 10 shown in FIG. 1 in its intended process eliminates the need for photo alignments between the source contact and the gate. Such alignments are generally critical steps in conventional MOSFET designs. Nevertheless, the structure 10 of FIG. 1 has two deficiencies. First, the entire active area of the surface of the device is subject to the BPSG etch back. The etching of the BPSG layer 18 can result in large areas on the surface that are subject to damage, defects and contaminants. Second, the gate-to-source capacitance is high due to the component of the polysilicon layer 22, BPSG layer 18 and the overlying source metal layer 12.
As a result, the process to formulate the structure 10 requires that both the BPSG and polysilicon recess etch must be accurately controlled. Otherwise, the devices will fail. Device failure will normally be due to the gate-to-source leakage if the BPSG 18 is over-etched. It is also possible that metal step coverage of layer 12 over the recessed area can be adversely affected by over-etching the BPSG layer 18. If the polysilicon layer 22 is over-etched, the device may fail because no inversion layer will form in the channel region. In addition, the BPSG etch that is used to open the source contact region is difficult to control due to the different substrate topography in the mesa trench and etch regions. Finally, those additional etches may cause additional damage and defects in the devices that could degrade the performance and the reliability of the devices.
The deficiencies and drawbacks of the structure 10 in FIG. 1 are overcome by the structure and the methods described herein. In the broader aspects of the invention, both the polysilicon and the BPSG recessed etches can be substantially minimized. Since neither the BPSG layer nor the polysilicon layer is excessively etched, there are less defects and damage to each layer. In addition, a high channel density and lower on-state resistance (RDSON) can be obtained by the methods of the invention. In a low-voltage trench MOSFET (typically less than 60 volts), channel resistance is a dominant component of the total RDSON. In this new structure and process, the source contact area is limited and the spreading resistance between the contact opening and source metal must be minimized. This is accomplished by forming a highly-conductive silicide such as titanium silicide or platinum silicide over the exposed source regions.